The invention relates to a method for the manufacture of fiber-reinforced moldings (SMC), in which the manufacturing procedure is performed in a resin mat forming apparatus, from the mixing of the individual components of the resin, up to the procedure of pressing moldings, in a continuous procedure, and it furthermore relates to an apparatus for the practice of the method.
On account of their good mechanical properties and low price, such moldings made by the SMC process are the most widely used thermosetting fiber-reinforced plastics.
Formerly, for the production of SMC parts, first the raw material was made, consisting of a resin mass (resin mixture) with embedded chopped glass fibers. This resin mixture is laid onto thin, transparent plastic films (carrier films) and subjected to a maturation process. The present-day maturation process takes around 4-5 days and serves to thicken the fillers added to the resin mixture. The resin mixture consists of resins, thermoplastic solutions, fillers, flow improvers, integrated parting agents, inhibitors and hardeners. It is stirred to form a well-dispersed liquid at a given temperature and on the resin mat apparatus it is also provided with a thickening agent in powder or liquid form, and spread with doctor blades onto the top and bottom carrier film and adjusted for width. The chopped glass from endless rovings, for example, is fed by chopping machines onto the bottom carrier film. Then the upper carrier film with the resin mixture is laid onto the chopped fiber coat. In a fulling process that follows, the fibers are mixed or imbibed with the resin mixture and then wound onto a roll as a resin mat or as a flat semi-finished product for the curing process. The ripening process makes the resin mixture so strong that the carrier films can be completely withdrawn, and in the extrusion process the glass fibers flow along with the matrix.
Shapes are cut from the flat semi-finished product or rolls to correspond to the molding being produced, and they are placed and pressed singly or in packs in a molding press.
The disadvantage of this method is that the SMC semi-finished product cannot be worked until after a long maturation period, thereby increasing the manufacturing costs.
It is also a disadvantage that the semi-finished product wound on a roll becomes distorted during the maturation process. If the roll lies on the floor, the pressure on the bottom SMC layers and hence a displacement of the material to the upper side. If the rolls are suspended on a shaft, the negative effect during the maturation is the opposite: the material flows downwardly and the layers of the material in the bottom part of the roll will have greater weights per unit area than the upper web layers. These unavoidably different weights per unit area make it necessary to cut the mats for each mold not just to length but also to weigh them so as to have equal bulk and thus equal part density. If the weights per unit area of the webs were constant, the necessary SMC web pieces could be cut automatically with a length gauge, which considerably reduces costs and would improve the quality of the moldings. This composition of the mats varying in thickness is also the reason why the manufacture of SMC moldings has never before been satisfactorily automated.
Another disadvantage of the former SMC manufacture is to be seen in the non-recyclable support films, which is another cost factor in the manufacturing process.